1. Field of the Invention
The invention relates to a test system for the post-production testing of an optical pick-up head control PCB for an optical disc drive.
2. Description of the Prior Art
Optical disc drives, of one form or another, have been considered standard equipment for personal computers for several years. Generally speaking, optical disc drives are used to read information stored on an optical disc. Examples of optical disc drives are known as compact disc drives (CD-ROM drives) and digital versatile disc drives (DVD-ROM drives) in the prior art. Some optical disc drives have the additional capability of being able to rewrite data onto an optical disc, for example CD-RW and DVD-RW drives. Optical disc drives are used in the playback and writing of many varieties of digital media including music, video, images, data archives, software and games.
FIG. 1 is a perspective view of a typical optical disc drive unit with ancillary components removed for clarity. Production of an optical disc drive 10 involves high precision manufacture, assembly and testing of at least the sub-modules detailed below:
11 An optical disc transport or traverse (TVS) assembly.
111 An optical pick-up head (OPU).
112 A spindle assembly.
113 A translation mechanism.
12 A disc loading mechanism.
13 A control printed circuit board (PCB).
14 A chassis.
Careful consideration must be given to matching certain sub-modules during assembly, for example, the performance of the TVS assembly 11 will depend heavily upon that of the control PCB 13, not least because it is responsible for the interface processing for the optical pick-up head. Optical pick-up heads utilize coherent light to read information from, and in some cases also to write information to, an optical disc. The coherent light source generally takes the form of a semiconductor laser diode, the power output of which must be closely controlled in any mode of operation. It is therefore essential that the driving and controlling circuitry of the optical disc drive control PCB 13 can adequately control laser power within predefined limits.
In the prior art, control PCBs have been tested by an arrangement utilizing a reference laser, light meter and various other test instruments, the optical disc drive control PCB being used to drive the reference laser while the laser output power and optical disc drive control PCB output to the laser are measured. By varying the optical disc drive control PCB output to the laser, and hence the laser output power, the control PCB output can be characterized and any adjustment made that may be required.
FIG. 2 shows a graphical representation of a typical optical disc drive control PCB output characterization 20. It can be seen that as more drive current 22 (generally expressed in mA) is supplied to the laser, the power output 21 (generally expressed in mW) of the laser will increase. Acceptable upper 25, and lower 24 limits of the ratio 23 of laser power output 21 to drive current 22 are prescribed for each test. By using a reference laser of known performance, the ability of the test PCB to control drive current within the predetermined limits can be judged against a required standard.
Without additional complexity though, this system has the limitation of testing only the open-loop characteristics of the optical disc drive control PCB, so naturally it is essential to also test closed-loop characteristics along with other functions, preferably together with the TVS assembly 11 that the particular control PCB will be matched to at final assembly into the optical disc drive 10. Indeed, this is common practice in the prior art, but other factors now effect the manufacture of optical disc drives as described below:
The above-mentioned sub-modules can, and generally are, manufactured at the same location, but as is increasingly the case in modern manufacturing practice, components and sub-modules may be outsourced or else shipped to different locations/customers if the manufacturer is acting as a first or second tier supplier to an Original Equipment Manufacturer (OEM). In such a situation, sub-modules such as the control PCB 13 cannot be tested together with the TVS mechanism 11 that they will eventually be embodied with into the completed optical disc drive 10, as they would during a single site manufacture and assembly process, because control of the sub-module selection and assembly process is relinquished to the customer.
It has become important then, to develop cost effective and reliable test methods to ensure that sub-modules, and in particular control PCBs, are operable and capable of performing within predetermined ranges when produced according to the above conditions, i.e. isolated from their destination assembly.
FIG. 3 shows a simplified schematic view of a typical optical pick-up head. Light from a laser 301 is collimated into a beam by a lens 302, a proportion of the light is reflected by a polarized plane of a polarizing beam splitter (PBS) 303 onto a photo-detector 304, while the remaining light is reflected by a mirror 307 and focused onto an optical disc 309 by an objective lens 308. Light reflected by the optical disc 309 follows a return path which is the reverse of that stated above, until the light reaches the PBS 303, where returning light is reflected by the polarized plane of the PBS 303, through a lens 305 and onto a photo-detector 306. The photo-detector 304 is dedicated to the task of sensing laser power, and while the primary task of the photo-detector 306 is to read disc information, it can be seen that both photo-detectors sense laser output and hence can be utilized for power detection. So in the prior art then, the means are found to both utilize a laser control drive current or signal, and to measure the laser emissions produced thereby.